Heat 1 tablespoon oil in a large heavy skillet over medium heat. Cook chicken until lightly browned, about 10 minutes. Reduce heat, and season with 1 teaspoon garam masala and cayenne. Stir in a few spoonfuls of sauce, and simmer until liquid has reduced, and chicken is no longer pink. Stir cooked chicken into sauce.

Mix together cornstarch and water, then stir into the sauce. Cook for 5 to 10 minutes, or until thickened.